It is always problematic in providing filter elements that such filter elements represent a flow resistance, which must be overcome by the user during breathing. This results in a burden for the user, which occurs in addition to the actual activity.
Such a filter element with a folded filter paper part is known, for example, from DE 694 12 268 T2. The described filter element has essentially a rectangular housing, into which a folded filter paper as the filter element is inserted. The folding of the filter paper takes place in this case, above all such that an as large as possible flow cross section is achieved, so that the flow resistance of the filter element is reduced. In addition to the above-described filter elements with a rectangular housing and a folded filter element, so-called ring filters, in which the folds of the filter are arranged concentrically, are, moreover, known. Ring filters are often used as particle-filtering elements especially for round housings. These ring filters consist of filter paper parts which are produced directly during the manufacture of the filter paper in the form of ring folds. Because of their design, ring filters are easy to seal in round housings in a particle-impermeable manner, for example, by clamping. However, a drawback of such ring filters is that because of their manufacture, wide folds and large fold distances and thus only a very small filter surface can be produced, so that the flow resistances and thus the resistances during breathing are comparatively great for the user.
For this reason, particle-filtering filters are often manufactured by corresponding filter elements being produced from pleated preforms of an element consisting of flat paper. In this case, the filter paper, which is, as a result, made available as roll material, is arranged into parallel folds in a zig-zag manner. The stability of the pleated preform is ensured by so-called spacers, which are connected to the folds of the filter element and hold same in their shape. Such pleated preforms are, as known from DE 69 412 268 T2, inserted into a rectangular housing and sealed against same by a connection being established between the frame and the outer edge of the pleated preform and the edge sealing thereof by means of hot glue.
Rectangular pleated preforms offer the advantage of an edge seal which can be comparatively easily produced, but are limited in this shape, namely, the rectangular shape, since the edge seal is only created at right angles to the running direction of the paper. Furthermore, the combination of traces of glue applied hot and edge seals and hot glue for the sealing of same in the housing of the particle filter for reasons of process safety is not optimal. The greater the gap between the filter element and the housing and thus the space provided for the seal is, the smaller is the usable filter surface.
In case of the use of round housings, as they are used, for example, in filters with RD-40 connection according to EN 148-1 or ISO 17420-3, round pleated preforms are cut out of the folded paper in order to utilize the maximum possible surface of the filter housing. In this case, the gap between the pleated preform and the housing is usually filled by casting or a glue is injected into the gap for the edge seal and sealing with the housing.
It is problematic in the prior-art particle filters and especially in the method for the manufacture thereof that during the manufacture of suitable filter elements, the sealing is carried out by applying an edge seal directly during the preparation of the pleated preform and, as a result of this, a part of the available filter surface is not usable. Further, large quantities of filter paper waste are often generated, which has to be discarded as scrap. This represents a considerable cost. Furthermore, it is often comparatively complicated in terms of production technology to create a reliable seal between the pleated preforms and the filter housings receiving same.